Water movement in the soil
Summary
TLDRThis educational video script explores the dynamics of water movement in soil, emphasizing the role of potential differences and soil structure. It uses time-lapse photography to illustrate capillarity, the forces of adhesion and cohesion, and how they counteract gravity. The script also discusses the impact of soil types, such as sandy and clay loams, on water infiltration and retention. It highlights how soil layers, such as coarse sand or clay pans, can act as barriers to water flow, affecting agricultural practices and soil management. The demonstrations serve to educate on the principles of unsaturated water flow, crucial for understanding irrigation, drainage, and soil conservation in agriculture.
Takeaways
- 💧 Water movement in soil is driven by potential differences, with water moving from areas of high potential to low potential.
- 🌱 The potential at any point in soil is influenced by forces such as gravity and capillarity, which cause water to move downward or upward.
- 🔍 Time-lapse photography in the video demonstrates how water moves into soil, showing processes that would take hours in nature in just minutes.
- 📏 The model used in the video, with soil between glass plates, represents a vertical cross-section of soil and allows for visualizing water movement.
- 🌿 Capillarity is a key principle where water moves into dry porous materials due to the attraction of solid mineral surfaces and cohesion of water molecules.
- 🏺 The height to which water rises in capillary action is influenced by the closeness of surfaces and the internal tension in the water.
- 🌳 Soil texture greatly affects water infiltration and retention, with sandy soils allowing more rapid infiltration but less retention compared to clay soils.
- 🌾 Sandy soils are better for irrigated areas due to their good infiltration properties, while clay soils are more suitable for dryland farming due to their water retention.
- 🚧 The presence of layers such as coarse sand or fine clay in soil can significantly impact water movement, acting as barriers or check valves.
- 🌱 The structure of soil, including the presence of aggregates and organic matter, plays a crucial role in water infiltration and the soil's ability to support plant growth.
- 🌱 Proper soil management, such as maintaining good tilth and avoiding compaction, can enhance water infiltration and reduce erosion.
Q & A
What determines the movement of water in soil?
-Water movement in soil is determined by the distribution of potential within the soil profile, with water tending to move from areas of higher potential to areas of lower potential.
What happens if there is no potential difference in the soil?
-If there is no potential difference, no water movement will occur.
What are the main forces that affect water potential at any point in the soil?
-The main forces affecting water potential are gravitational and matric forces, which cause water to move downward due to gravity and upward due to the attraction of solid surfaces, known as capillarity.
How does the soil model in the video represent a real-world scenario?
-The soil model, held between glass plates, represents a vertical cross-section of soil, allowing viewers to observe water movement as it would occur in nature, albeit accelerated due to time-lapse photography.
What principle is demonstrated when water rises between two closely spaced plastic plates?
-The principle of capillarity is demonstrated, where water is pulled upward against gravity due to the adhesive forces between the plastic and water and the cohesive forces between water molecules.
Why does the height of water rise differ when plastic plates are pinched together?
-The height of water rise is greatest when the plastic plates are pinched tightly together due to the increased adhesive forces and the resulting internal tension in the water.
How does the size of soil pores affect the rate of water flow and retention?
-The finer the soil pores, the more restricted the rate of water flow, and the greater the water retention. Sandy soils have larger pores and allow for greater initial penetration, while clay soils with finer pores retain water more effectively.
What is the significance of soil texture in irrigation and agricultural practices?
-Soil texture is significant because it affects the infiltration properties and water-holding capacity, which in turn influence irrigation needs and the suitability of the soil for different agricultural practices.
How does a layer of coarse sand in soil affect water movement?
-A layer of coarse sand can act like a check valve, holding water back until the overlying soil becomes very wet, at which point it allows excess water to pass through, demonstrating the principle of unsaturated flow.
What is the impact of a fine clay layer within otherwise uniform soil?
-A fine clay layer can restrict root growth and water penetration, often leading to water table buildup above such layers. It can impose limitations on agricultural use due to its resistance to water flow and its impact on soil drainage.
How do soil aggregates and their pores influence water movement?
-Soil aggregates with large pores can transmit water readily under saturated conditions, but under unsaturated conditions, water moves into and through the soil due to the attraction of solid surfaces, with the rate of movement being restricted by the number of contacts between aggregates.
Outlines
💧 Water Movement in Soil
This paragraph discusses the principles governing water movement in soil. Water in soil moves due to potential differences, influenced by forces like gravity and capillarity. The video uses a model with soil between glass plates to demonstrate these principles. The model represents a vertical cross-section of soil, and time-lapse photography is used to speed up the observation of water movement. The soil used in the demonstration is air-dried loam with sand, clay, and aggregates to simulate non-uniform bodies. The video illustrates capillarity, where water is pulled into dry porous materials due to the attraction of solid mineral surfaces. The height of water rise is greatest when plastic plates are closely spaced, demonstrating tension in the water. The paragraph also compares the water retention and transmission abilities of sandy and clayey soils, explaining their suitability for different agricultural practices.
🌱 Soil Stratification and Water Flow
This paragraph explores how water enters the soil and the role of soil stratification in water movement. It emphasizes that water movement is influenced more by the attraction of solid surfaces than by gravity alone. As the soil becomes wetter, gravity plays a more significant role. The video demonstrates how a layer of coarse sand acts as a check valve, holding water back until the soil is very wet. The paragraph also discusses how soil layers, such as a clay pan, can restrict root growth and water penetration, leading to water table buildup. The text compares different soil types and their impact on drainage and plant growth, using examples from agricultural land in Belgium. It concludes by explaining how the principles observed in the model apply to field soils with layers of sand and gravel.
🌾 Water Flow and Agricultural Practices
This paragraph delves into the practical applications of water flow principles in dryland agriculture. It explains how water movement in soil is affected by the size of soil pores and the presence of free water or water under pressure. The video shows how soil aggregates and their contact points influence water movement, with large pores facilitating rapid water entry under saturated conditions. The paragraph contrasts the effects of organic matter on soil structure, with well-aggregated soil allowing for better water infiltration compared to soil with channels or cracks. It also discusses the placement of tile drains for effective water management, emphasizing the need for them to be below the water table in wet soils. The video concludes by demonstrating how certain soil conditions, such as a straw layer, can hinder water infiltration and increase soil erosion, highlighting the importance of understanding water flow principles for sustainable agriculture.
🌱 Unsaturated Flow and Agricultural Land
This final paragraph summarizes the principles of unsaturated water flow in soil and porous materials. It reiterates that water movement is driven by the attraction of solid surfaces for water and the cohesive forces between water molecules. The paragraph emphasizes that the nature of water movement is dependent on the characteristics of the soil pores and the porosity of the soil. The video demonstrates how water moves differently under unsaturated conditions compared to saturated conditions, with the former being influenced by the tension in the soil. The paragraph concludes by stating that these principles are applicable to agricultural land where crops are grown, and understanding them is crucial for effective water management in farming practices.
Mindmap
Keywords
💡Water movement
💡Potential
💡Capillarity
💡Adhesion and Cohesion
💡Soil texture
💡Infiltration
💡Stratification
💡Water table
💡Aggregates
💡Erosion
💡Unsaturated flow
Highlights
Water movement in soil is driven by potential differences, with water moving from higher to lower potential areas.
The potential at any point in soil is influenced by gravitational and matric forces, causing water to move downward due to gravity and upward due to capillarity.
A time-lapse photographic process is used to observe water movement in soil, accelerating natural processes for easier viewing.
Demonstrations show water rising between plastic plates due to adhesive and cohesive forces, illustrating the principle of capillarity.
The height of water rise due to capillarity is greatest when plastic plates are closely spaced, demonstrating the effect of tension.
Different soil textures (sand, silt, clay) exhibit varying abilities to transmit and retain water, with sand allowing the deepest penetration and clay the best retention.
Sandy soils have good infiltration properties but poor water retention, making them suitable for irrigated areas.
Clay soils are difficult to irrigate due to low infiltration rates but are suitable for dryland farming due to their water retention capabilities.
The rate at which water enters the soil is crucial for designing irrigation systems and cultural practices for erosion control.
Water movement in soil is primarily due to the attraction of solid surfaces rather than gravity, especially when the soil is not completely saturated.
A coarse sand layer in soil acts like a check valve, holding water back until the soil above is very wet, then allowing excess to pass through.
Soil layers with sand and gravel can affect drainage and plant growth, as seen in Belgian soils with loam overlying brolan sands.
A fine clay layer in soil can restrict root growth and water penetration, leading to water table buildup if present at shallow depths.
The resistance to water flow in fine pores of restrictive layers is so great that little water is transmitted through them over weeks or months.
Coarse materials with large pores aid in water movement only under conditions of free water or positive water pressure.
Principles of water flow are applied to Dryland agriculture, showing the importance of proper soil tilth and organic matter for water infiltration.
Channels or cracks in soil do not assist in water movement unless connected to a source of free water, which is also true for tile drains.
A straw layer can check downward water flow, leaving more water to run off the surface and making the soil more vulnerable to erosion.
Unsaturated flow of water in soil is driven by the attraction of solid surfaces for water and the cohesion of water molecules.
Transcripts
water movement in a soil is determined
by the distribution of potential within
the soil
profile water tends to move from areas
of higher potential to ones of lower
potential
if there is no potential difference no
movement will occur the potential at any
point results from the action of
different forces mainly gravitational
and matric water tends to move downward
due to gravity and upward due to
attraction of Solid Surfaces called
capillarity through a series of small
experiences this video will show the
basic principles governing water flows
in these demonstrations soil is held
between glass plates so that you can
watch what happens as the soil is wetted
the glass plates are 30 cm high and 60
cm wide with about 2.5 cm of space
between for soil think of this model as
representing a vertical cross-section
through the soil thanks to time-lapse
photographic processes actions that
would require many hours in nature will
be observable in just a few minutes
using a motion picture camera single
pictures are taken every second as the
water moves into the soil the completed
Motion Picture film is then projected
and speeded up 25
times because water movement is usually
very rapid when water is first applied
and very slow at later times the speed
up Factor might change during a sequence
this will be indicated on the screen the
soil used is an air dried LOM which has
been passed through a fine screen sand
clay and Aggregates are used to simulate
non-uniform bodies in a salt profile
water will be added in a Farrow with a
funnel connected to a to which keeps the
water level at any desired
depth before the sequences using
time-lapse photography here is a
demonstration that illustrates the
principle of
capillarity this principle is involved
when water moves into dry porous
materials liquid is pulled because of
the attraction of solid mineral surfaces
for water adhesion and attraction of
water molecules for each other cohesion
adhesive and cohesive forces are
responsible for moving water upward
against the downward force of gravity in
this demonstration water rises between
two closely spaced plastic plates
because of the adhesive forces between
plastic and water and cohesive forces
between water
molecules the height of Rise is greatest
when the plastic plates are pinched
tightly
together the pressure in the water above
the free water surface in the container
is less an atmospheric pressure this is
called
tension the higher water rises the
greater the internal
tension now to the models and time-lapse
pictures
before analyzing the problems of
stratification it is important to
illustrate differences among uniform
Souls with respect to their ability to
transmit and retain
water note that the depth of penetration
at any given time is greatest for the
sandom which has the largest pores and
least for the clay which has the finest
pores the finer the pores the more the
rate of water flow is
restricted retention of water after the
source is removed is greatest in a clay
Loom which has the finest pores and
least in a Sandy Loom despite this
however the net useful storage is
greatest in a clay Loom and least in a
Sandy Loom although Sandy Loom retains
less use for water than does the clay
LOM it is a good soil in an irrigated
area where lack of water holding
capacity can be compensated by
irrigation the infiltration properties
are generally good clay Looms on the
other hand are often difficult to
irrigate because of low infiltration
rates in dry climates where there is no
irrigation and Sandy Loom would not hold
enough water to carry agricultural
plants through the growing season a clay
Loom by contrast would retain more water
over a longer period of time hence
dryland farming on fine Tex Ed soils is
practical the rates at which water would
enter the soil is an important factor to
consider when designing an irrigation
system or when deciding on cultural
practices for use and erosion control
watch as the water moves out from an
irrigation Farrow note that the movement
outward is almost as great as that
downward this is added evidence that the
force responsible for this type of water
movement is mainly due not to
gravitation but to the attraction of
Solid Surfaces as the soil becomes
wetter and wetter however gravitation
plays a stronger role and if the soil
becomes comes completely saturated then
gravitational forces
predominate the horizontal layer you see
is coarse sand one of the important
principles of unsaturated flow is
described as you witness what happens as
the wedding front Encounters this layer
of coarsed
sand the pores in the soil are many
times smaller than those between sand
grains water is held in these small
pores by large adhesive and cohesive
forces the pores in the oil are like the
pores in a piece of blooding paper used
to soak up ink the huge pores in the
sand cannot hold water at the tension
which exists in a wetted soil above so
the water does not move readily into the
sand however as the soil above the sand
becomes very wet the water eventually
moves into the sand just as ink would
drip from a bladder which is wet
excessively the sand layer does ACT
something like a check valve holding the
water back until the soil becomes very
wet and then letting the excess pass
through what happens to water and soil
containing a sand layer is typical in
principle of what happens to water in
field soils where sand and graval occur
as layers in finer soil materials a
great deal of agricultural land is
layered in this fashion in Belgium you
can find soil composed of lome overlying
brolan Sands in the woon Bren Province
this layout greatly affects the drainage
and the ability to support plant growth
as more water can be retained this is
one of the best soils in our
regions now in this sequence you see a
layer of fine clay in otherwise uniform
soil this clay layer is similar to a
clay pan or any type of layer in which
the pores are extremely fine compared to
the pores in the overlying
soil these layers often restrict root
and depths of plants and are
particularly known for the trouble they
cause in preventing downward penetration
of water when excess water is added to
the soil water tables are often built up
over such layers if they occur at Shel
low depth water tables often rise above
the land surface during wet Seasons
imposing serious limitations on
agricultural
use despite the fact that a clay pan
hinders downward movement of water it
does absorb water readily as the soil
above is wetted observe the wetting
front as it moves into the clay pen the
pores in the clay are much finer than
those in the overlying soil so they can
pull water from the soil water tables
are not built up over clay pans because
of the inability of water to enter them
instead water tables result from slow
transmission of water the resistance to
water flow in these extremely fine pores
of layers like these is sufficiently
great that even over periods of weeks
and months little water is transmitted
through them into the soil
below the poison restrictive layers
found in nature are quite variable they
range all the way from fine pores that
allow almost no water to pass up to
pores that are almost as large as those
of the overlying soil the extent to
which downward flow is restricted and
water storage is altered depends on the
Finesse of these pore and the thickness
of the restricting
layer this is in contrast to what was
shown earlier in soil overlying Co sand
layers there the downward movement of
water was temporarily checked but water
tables could not be built up so as long
as their opportunity for free drainage
into the course materials was
possible this model has a sun layer on
the left and the layer of Cor Aggregates
on the right the pores between sand
grains and those between Aggregates are
large but Aggregates are made up of soil
particles like those of the surrounding
soil water movement in soil materials
which wet readily depends upon paracity
and not upon the chemical or
minerological nature of the so material
unless it influences its
porosity each individual soil aggregate
contains numerous fine pores of a size
similar to the pores in the surrounding
soil as water approaches these
Aggregates note as they wet out as soon
as the water reaches them however pores
between Aggregates are too large to hold
water at the existing tensions hence
they remain empty all the water must
therefore move first to the finer pores
of an Aggregate and then across the
point of contact with the adjoining
Aggregates the small number of contacts
between Aggregates restricts the rate at
which water can move
if free water is supplied directly to a
layer of coarse sand water rushes in
rapidly filling all the pores these are
conditions of saturated flow the moving
force is due to positive pressure from
the water in the Farrow under saturated
conditions large pors can transmit water
readily with a rate of transmission in a
given material depending only upon the
hydrostatic pressure of the water supply
the energy derived from this positive
pressure is dissipated rapidly over very
short distance in a fine pores giving
way to absorb these forces in a drier
soil water moves out into the soil from
the sand layer under unsaturated
conditions
it is pulled into and through the soil
because of the attraction for water of
the middle surfaces making up the fine
pores of the
soil the sand in the layer at the left
is the same kind of sand through which
water is Flowing at the right here
however the layer is not in contact with
free water or water under positive
pressure the surrounding soil is wetted
under unsaturated conditions where the
water is present only under tension this
sand layer cannot wet until the water
tension in the surrounding soil becomes
very low which means that the soil
becomes very wet as this happens the
layer takes water coarse materials with
very large pores Aid in water movement
only under conditions where a contact
free water or water under pressure where
water exists only under tension such
materials stop or materially
water flow
the Practical applications of principles
of water flow to Dryland agriculture are
shown here water moves rapidly into cils
with good
D proper til practices on the soil on
the left have produced numerous small
Aggregates which have been stabilized by
decomposing organic materials the
resulting large pores which remain open
all the way to the surface take water
readily thus the infiltration rate
remains High the same amount of organic
material when turned under in a layer
does little to improve soil to and if
anything makes conditions worse on the
right a channel filled with coarse sand
simulates an open Channel left by
borrowing worms or angle worms or
perhaps a channel left by decaying roots
of straw
such channels or cracks do not assist in
water movement when they are not
connected to a source of free
water the principle involved also
applies to tile drains water can move
into such drains only if positive water
pressures exist in the surrounding soil
hence stle drains placed in wet soils
must be located below the water table if
they are to carry away unwanted water
and make the land
tillable the straw layer like a sand
layer checks downward flow of water in
this case not only does less of the
rainfall penetrate into the root Zone
leaving more water to run on the surface
but wet conditions in a plot Zone make
the soil even more vulnerable to damage
such as that caused by the impact of
falling
raindrops thus you can understand why
soil and water loss by erosion is
accelerated
these demonstrations emphasize the
principles of water flow under
unsaturated conditions conditions under
which crops are grown on agricultural
land each demonstration has its
counterpart in nature where it may be
less dramatic but the principles hold
and can be seen in operation if one
observes
carefully in summary the then
unsaturated flow of water in soil and
other porous material takes place
because of the attraction of solid
surfaces for water and of water
molecules for each other how the water
moves depends upon the nature of the
pores and the porosity changes in the
porous system
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